There is no major reason for this but just to make your blog outstanding and different from other blogger blogs And make it look beautiful with a customized anonymous avatar.
To Get This done, follow this steps below:
#1. Goto Your Blogger Blog Dashboard.
#2. Locate the blog you want to customize it on
#3. Goto Template
#4. Click Edit Html (Remember to Backup Your Blog Template incase you Make a mistake
#5. Click "Expand Widget Templates" box
#6. Find </body> Add the code below just before </body>
Change AvatarUrl To The Url Of Your New Avatar You Want To Use.
Save Your Template and You are Done.